1. Field of the Invention
The present invention relates to a process for the preparation of trimerized polyisocyanates that contain iminooxadiazinedione groups in the presence of a quaternary phosphonium polyfluoride trimerization catalyst.
2. Description of the Prior Art
Polyisocyanates containing iminooxadiazinedione groups (asymmetric trimers) are valuable, high quality raw materials, which may be used, e.g., for the manufacture of polyurethane lacquers and coatings (e.g. DE-A 19,611,849). These polyisocyanates are present as a subsidiary component in the well known polyisocyanates containing isocyanurate groups (symmetric trimers).
Isocyanate oligomers having a significantly increased iminooxadiazinedione content are the subject of DE-A 19,611,849. Their advantageous properties, for example, as a raw material for the manufacture of polyurethane lacquers and coatings, are described. For (di)isocyanate oligomers having at least three NCO groups, polyisocyanates containing iminooxadiazinedione groups have the lowest viscosity.
The preparation of isocyanate trimers containing iminooxadiazinedione groups using ammonium polyfluoride catalysts is described in the examples of DE-A 19,611,849. When this process was transferred from laboratory scale to industrial scale, it was found that the proportion of asymmetric trimer in the trimer mixture varied. In this application the term "trimer mixture" is defined as the sum of symmetric trimers (isocyanurates) and asymmetric trimers (iminooxadiazinediones). The products which can be prepared in that manner may occasionally also exhibit a high level of turbidity (greater than 1.5 TE(F) when measured using a device from Hach).
Thermokinetic studies of the trimerization reaction of hexamethylene diisocyanate (HDI) using ammonium polyfluoride catalysts in a reaction calorimeter (for the measuring arrangement and principle see J. Thermal Anal. 1983, 27, 215-228) showed that in some tests the progression of the evolution of heat with time differs greatly from the usual pattern. The general pattern is increased production of heat after addition of the catalyst and then a more or less slow but steady fall in the heat of reaction as a result of deactivation of the catalyst in the reaction mixture, which is caused by thermal decomposition and reaction with trace impurities in the isocyanate starting material.
In contrast, in many cases the expected rapid release of heat of reaction occurred first, after which the reaction rather untypically died down rapidly and then started up again. Surprisingly, the addition of further catalyst did not immediately accelerate the reaction, but rather the reaction slowed down for a short time immediately after addition of the catalyst and then, after passing a minimal heat production rate, accelerated again for no obvious external reason as shown in Example 2 and FIG. 1.
However, this phenomenon is not observed in all cases. Nor is it dependent on the reaction temperature. If no abnormal progression of the heat production curve with time is observed, the proportion of asymmetric trimers is at the same high level achieved in laboratory tests (i.e., over 30 mole % in the trimer mixture). If the above-mentioned abnormal progression of the heat production curve is observed, products having a much lower iminooxadiazinedione content are obtained.
Obviously, in a scarcely foreseeable manner, the actual catalytically active species, which yields different products according to the type of reaction (normal versus abnormal in the sense of the preceding description), forms only during the reaction from the ammonium polyfluoride that is added, as a result of the effect of the isocyanate to be oligomerized or the secondary products present in these isocyanates.
This circumstance renders considerably more difficult the specific, reproducible industrial manufacture of high quality lacquer polyisocyanates having reproducible properties such as viscosity, NCO content, color index, turbidity, etc.
An object of the present invention is to provide a reproducible process which is not subject to the above-mentioned incalculabilities such that
1) it is possible to carry out the reaction in a foreseeable manner in direct dependence on the amount of catalyst used, PA1 2) the heat produced in the exothermic reaction is to occur uniformly and be removable uniformly and PA1 3) it is to be possible to prepare products having an expected, uniform composition and quality. PA1 R represents identical or different, optionally branched aliphatic, aromatic and/or araliphatic C.sub.1 -C.sub.20 groups, or two or more R groups may also form, with one another and with the phosphorus atom, saturated or unsaturated rings and PA1 n has a value of 0.1 to 20.
This object may be achieved by the process according to the invention by catalyzing isocyanate trimerization with quaternary phosphonium polyfluorides.